Your post is not entirely clear. Is this what you said?

1. You closed on a brand new home on December 31, 2020, and moved in right away.

2. When you moved in, carpet in some bedrooms was soaked.

3. You complained to the builder, he identified the problem, fixed the leak and dried the floors.

4. Your husband hid something under one of the carpets in the spring of 2021.

5. In the fall of 2022, you removed the thing from on/under the carpet and it was moldy.

6. Now you want to know if you can ”do” something. What did you have in mind to ”do” exactly? You mean to the builder?

That explains why " you have to keep the money under the mattress" this way it's off the floor, fewer chances of getting wet. :-)

First thing I'd do is figure out the extent of the problem. I'd call a few home inspectors, explain you suspect moisture from an old leak, and ask what it would cost for them to help you pinpoint the problem. (I have no idea how much they'd charge, sorry.) You could also look up "moisture meters for home mold" and follow few links, see if that helps.

A no-cost method that might be useful: put some tissue under a plastic bin elsewhere in your home, and see if that feels damp.

Assuming there is a problem found (that is, more moisture elsewhere in your house) I really have no idea what to do to either fix it or get the seller to pay anything. Maybe someone else will chime in.

The great thing is you can clean the money with a little bit of bleach in a lot of water. But it sounds like you need to pull up the carpet and see what is going on. And I would 100% contact the builder if there is mold under there, and I would see if the pipe is still slowly leaking. Maybe the fix wasn’t very good. Do you have a paper trail of the original issue? I would also contact my insurance company. That is what they are for. They may decide to go after the builder if you have a record of the original issue, but otherwise they should cover it. Of course, you have your deductible

Is the area around the hiding place damp? Maybe the $$ was wet when he bagged it?

"Assuming there is a problem found (that is, more moisture elsewhere in your house) I really have no idea what to do to either fix it or get the seller to pay anything. Maybe someone else will chime in."

Chiming in. They might have neglected to put down a moisture barrier when foundation was poured.

Did you run a dehumidifier for a while after the repairs were finished? Are you running one now?

Guessing from your post that you didn't do a walk through before the closing.