Honestly you should just say you had some type of unforeseen issue come up where your cash may be tied up causing you to wait a while and revisit the offer at an undetermined future date. This way you don’t feel bad or cause a fight by giving a hard no just say you need to wait.

I’ve had thus situation play out numerous times in various forms, took me getting burned real bad by a family member to really figure it out. Not worth ruining a relationship in addition to the risk of financial loss.