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If you get offered a non-paid position with a government agency and have an interview for a paid position coming up the next week which you think you have a good shot at, what is the proper protocol for trying to keep the unpaid offer open until you hear back from the paid position?

Dear _________,

Thank you for your offer to work at ___________ this summer. I am still interviewing with other employers, but I will get back to you no later than ___________. If you need an answer from me before this time, please let me know.

Best Regards,

______________