What can I do with the framed dollar bill in Gio's?

As long as it's still on the wall, not much.

Have you listened to your ally's VOICE MESSAGES? If not, come back after you have.

Do you notice any changes?

The framed dollar bill "somehow" fell on the counter, leaving behind some subtle claw marks. (We're looking at you, Gio.)

Pick up the dollar and give it a closer look.

Can you work out the numbers that are missing on the bottom right?

Maybe the note accompanying it holds a hint?

(PARTIAL SOLUTION FOLLOWS)

↓

Multiply the digits of each double-digit number on the top left, to get the respective two-digit number on the bottom right.

The missing numbers are 12 and 28.

Now what to do with those numbers.

Where would you put a dollar bill?

Especially if it had been used to buy something?

How about the cash register?

(SOLUTION FOLLOWS)

↓

Press $10, $2, .20 and .08 on the cash register, then the black button on top to ring the sale.

Take the key from the secret compartment.