I don't get it. Why did he pawn the ring?
Inaccurate translation.
Because of how casual her speech is (and because Japanese has a habit of omitting subject), the girl can sound both
"If we're still single by 30, let's get married."
and
"If I'm still single by 30, I'll just marry someone random."
Coz of that, the guy thought she didn't want him.
So the joke was
P1. "If we're still single by 30, let's just settle and get married"
P2. "O... okay then..." (girl thought she baited him perfectly)
P3. (girl surprised he's actually had bought a ring)
p4. (boy pawning off the ring because he thought she already has someone else in mind)