exchange_router: cancel_order does not refund execution fee to user

[abayomicornelius]

Summary

When a user calls exchange_router::cancel_order, the collateral token is returned but the execution fee (paid in XLM/native token to incentivise the keeper) is silently retained in the order_vault. Users lose the execution fee even though no keeper performed any work.

Expected Behaviour

On user-initiated cancellation, the full execution fee should be returned to the user. The keeper fee is only earned upon a successful (or failed-but-attempted) execution.

Steps to Reproduce

1. Create an order with execution_fee = 1 XLM
2. Call cancel_order before keeper executes
3. Check user wallet — collateral returned, 1 XLM missing

Acceptance Criteria

• cancel_order transfers execution fee back to env.invoker()
• Integration test: order created with fee, cancelled, assert full balance restored
• Keeper-initiated cancellation (via execute_order_fail) still pays keeper the execution fee
• No change to behaviour when execution_fee = 0

Affected Contract

exchange_router, order_vault

[Hassanmovic98]

I'll fix the execution fee retention bug. When a user cancels their own order, the full fee gets returned — the keeper didn't execute anything so they shouldn't keep the fee.

Keeper-initiated cancellation (failed execution) still pays the keeper because they attempted the order. Zero-fee orders unchanged.

Integration test: create order with fee, cancel it, verify the full balance is restored to the user. Exchange router updated to transfer the fee back on user cancellation.

[Hassanmovic98]

I'll fix the execution fee retention bug. When a user cancels their own order, the full fee gets returned — the keeper didn't execute anything so they shouldn't keep the fee.

Keeper-initiated cancellation (failed execution) still pays the keeper because they attempted the order. Zero-fee orders unchanged.

Integration test: create order with fee, cancel it, verify the full balance is restored to the user. Exchange router updated to transfer the fee back on user cancellation.

[JudeDaniel6]

@JudeDaniel6 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello Maintainer, Please can i work on this issues and give you a better result

ℹ️ Repo Maintainers: To accept this application, review their application or assign @JudeDaniel6 to this issue.

[Oluwasuyi-Timilehin]

@Oluwasuyi-Timilehin has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello @maintainer, I’d like to work on this issue. I have experience handling similar tasks and can start immediately. And I will deliver at the shortest possible time.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Oluwasuyi-Timilehin to this issue.

[kulayddon]

@kulayddon has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I participated in the project last wave, so I'm already familiar with how things are structured here. This issue fits what I do well — I'll implement it cleanly, add tests, and keep the PR easy to review. Happy to start right away.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @kulayddon to this issue.

[grantfox-oss]

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Next steps:

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Congratulations, @kulayddon! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@kulayddon's PR #349 was approved and merged by @abayomicornelius.

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👏 Great work, @kulayddon! Keep contributing to SO4-Markets.