diff --git a/src/empire/empire.controller.ts b/src/empire/empire.controller.ts
index 3025ba1..dbee13e 100644
--- a/src/empire/empire.controller.ts
+++ b/src/empire/empire.controller.ts
@@ -1,5 +1,5 @@
-import {Body, Controller, ForbiddenException, Get, Param, Patch} from '@nestjs/common';
-import {ApiForbiddenResponse, ApiOkResponse, ApiOperation, ApiTags, refs} from '@nestjs/swagger';
+import {Body, Controller, ForbiddenException, Get, Param, ParseBoolPipe, Patch, Query} from '@nestjs/common';
+import {ApiForbiddenResponse, ApiOkResponse, ApiOperation, ApiQuery, ApiTags, refs} from '@nestjs/swagger';
 import {Auth, AuthUser} from '../auth/auth.decorator';
 import {User} from '../user/user.schema';
 import {Throttled} from '../util/throttled.decorator';
@@ -48,6 +48,12 @@ export class EmpireController {
 
   @Patch(':empire')
   @ApiOperation({description: 'Update empire details.'})
+  @ApiQuery({
+    name: 'free',
+    description: 'Change the resources directly without spending or receiving credits. ' +
+      'Useful for events, testing or debugging. ' +
+      'Note that it is not possible to change the population directly.',
+  })
   @ApiOkResponse({type: Empire})
   @ApiForbiddenResponse({description: 'Cannot modify another user\'s empire.'})
   @NotFound('Game or empire not found.')
@@ -56,12 +62,13 @@ export class EmpireController {
     @Param('game', ObjectIdPipe) game: Types.ObjectId,
     @Param('empire', ObjectIdPipe) id: Types.ObjectId,
     @Body() dto: UpdateEmpireDto,
+    @Query('free', new ParseBoolPipe({optional: true})) free = false,
   ): Promise<Empire | null> {
     const empire = await this.empireService.find(id) ?? notFound(id);
     if (!currentUser._id.equals(empire.user)) {
       throw new ForbiddenException('Cannot modify another user\'s empire.');
     }
-    this.empireService.updateEmpire(empire, dto);
+    this.empireService.updateEmpire(empire, dto, free);
     await this.empireService.saveAll([empire]); // emits update event
     return empire;
   }
diff --git a/src/empire/empire.dto.ts b/src/empire/empire.dto.ts
index 31a4c09..25591ae 100644
--- a/src/empire/empire.dto.ts
+++ b/src/empire/empire.dto.ts
@@ -5,6 +5,7 @@ import {SYSTEM_TYPES, SystemTypeName} from '../game-logic/system-types';
 import {IsIn, IsOptional} from 'class-validator';
 import {PartialType} from '../util/partial-type';
 import {RESOURCES_SCHEMA_PROPERTIES} from '../game-logic/types';
+import {ResourceName} from '../game-logic/resources';
 
 export class ReadEmpireDto extends OmitType(Empire, [
   'resources',
@@ -43,7 +44,9 @@ export class UpdateEmpireDto extends PartialType(PickType(Empire, [
 ] as const)) {
   @ApiPropertyOptional({
     ...RESOURCES_SCHEMA_PROPERTIES,
-    description: 'Update resources for market trades. The credits are automatically updated as well.',
+    description: 'Update (delta) resources for market trades. ' +
+      'The credits are automatically updated as well (unless the `free` query parameter is true). ' +
+      'Use negative values to sell resources, positive values to buy resources.',
   })
-  resources?: Record<string, number>;
+  resources?: Record<ResourceName, number>;
 }
diff --git a/src/empire/empire.service.ts b/src/empire/empire.service.ts
index 1a29f75..2770441 100644
--- a/src/empire/empire.service.ts
+++ b/src/empire/empire.service.ts
@@ -1,4 +1,4 @@
-import {Injectable} from '@nestjs/common';
+import {BadRequestException, Injectable} from '@nestjs/common';
 import {InjectModel} from '@nestjs/mongoose';
 import {Document, Model} from 'mongoose';
 import {EventRepository, EventService, MongooseRepository} from '@mean-stream/nestx';
@@ -9,6 +9,7 @@ import {COLOR_PALETTE, EMPIRE_PREFIX_PALETTE, EMPIRE_SUFFIX_PALETTE, MIN_EMPIRES
 import {generateTraits} from '../game-logic/traits';
 import {Member} from '../member/member.schema';
 import {EmpireLogicService} from './empire-logic.service';
+import {ResourceName} from '../game-logic/resources';
 
 @Injectable()
 @EventRepository()
@@ -40,11 +41,20 @@ export class EmpireService extends MongooseRepository<Empire> {
     return rest;
   }
 
-  updateEmpire(empire: EmpireDocument, dto: UpdateEmpireDto) {
+  updateEmpire(empire: EmpireDocument, dto: UpdateEmpireDto, free: boolean) {
     const {resources, ...rest} = dto;
     empire.set(rest);
     if (resources) {
-      this.empireLogicService.tradeResources(empire, resources);
+      if (free) {
+        for (const resource of Object.keys(resources) as ResourceName[]) {
+          if (resource === 'population') {
+            throw new BadRequestException('Cannot directly change empire population.');
+          }
+          empire.resources[resource] += resources[resource];
+        }
+      } else {
+        this.empireLogicService.tradeResources(empire, resources);
+      }
     }
   }