The simplest way would be to say "in the next few days" if you really want it back within that time period. If it's not too urgent, you can say "I do hope you'll pay me back soon"

In your example, saying, "Sometime in the near future" would be a common response. "I hope you will pay me back sometime in the near future". You could also say, "I hope that you will pay me back sometime soon".

Вы можете использовать in the nearest future / in the very near future или более формальные at the earliest opportunity / at the earliest possible date. Думаю, in the nearest future самая широко используемая фраза.