Keyshawn Johnson -- Pays for $60 Sweatshirt ... With I.O.U. Note

When you're Keyshawn Johnson ... your word is as good as cash -- so says an AZ woman who claims the ex-NFL star paid for a $60 Super Bowl sweatshirt with an I.O.U. note.

We're told Key was leaving his Scottsdale hotel Friday night -- when a rainstorm broke out and Key realized he didn't pack the proper attire.

So, he went up to a lady (Sarah) selling SBXLIX swag and tried to buy a sweatshirt. Problem is, he didn't have any cash on him at the time.

But Key had a solution -- he wrote an I.O.U. note for the gear ... and signed it with his name, the debt and his old jersey number, #19.

Now, we've learned ... Key has been in contact with Sarah -- and has told her he intends to pay back every single cent.

"We finally got in touch and he's going to just wire the money to my account," Sarah tells us ... "I'm happy! I knew he'd be good for it."