Tenant wants rent back

[Kurt K.]

Had a tenant pay in full for the month (with late fee)

Got a text and girl said her husband was only supposed to give me $50 less because they needed it for groceries this week.

What they paid me is what they owed for the month, should I give them $50 back and collect next week? These tenants are late every month, but pay rent and late fee. They are currently paid in full for December (as of today).

What would you do? What would you say back to them if you keep it?

I know it is rightfully mine, but I think I am rather "soft" haha