No, this is very common! It's not MAKE a favour, it's DO a favour.

不对，这很常见！不是 MAKE a favour，这是 DO a favour。

It's quite common, if someone agrees to do a favour, to use the future tense to promise a reward.

如果某人答应帮你什么忙，我们用将来时许诺一定的谢礼，这是很常见的。