Do fuel pumps have a device that lets one decide how much to spend?

Yes some places you can tell the attendant how many dollars to pump.

They all do. You just prepay the clerk.

The pumps don’t, to be exact. It’s the electronic stuff on the console that the cashier has. The cashier enters:
a) type of gas (diesel, unleaded, super unleaded, etc.)
b) pump number
c) maximum $ amount before pump stops

When you go to fill the tank at the pump number, it will let the gas flow up to the dollar amount. In my experience it is inexact – if you say $20.00, it may not click off until $20.02 or $20.03 or so.

Whoo Hooo…found it
On the keypad on the pump, press & hold the * & the # at the same time. The screen will ask for the Preset Amount. Enter it & press OK. Then pump your gas.

I’ve NOT tried it yet, but you better believe I will try it the next time I’m getting gas. :-)

Yes. Back in the day we got together 50 cents just to make it home once.

Yes, with the new style pumps. Just tell the clerk how much you want and the pump will shut off at that exact gallon or dollar or any price down to the penny.