KevinForstereditingguy can help you out.
I dont know how great your skills are, but you have to:
Init.sqsinit.sqsyourcash = 100
priceredcar = 400
redcar lock true
buy the redcarbuyredcar.sqs? (yourcash - priceredcar) >=0 : redcar lock false; yourcash = yourcash - priceredcar
[] exec "leavecarshop.sqs"
[] exec "carshop.sqs"
Entering the carshopcarshop.sqsplayer sidechat format ["You have £%1", yourcash]
player sidechat format ["The Sport car costs £%1", pricesportcar]
player sidechat format ["The Red car costs £%1", priceredcar]
buyredcar = player addaction ["Buy red car", "buyredcar.sqs"]
If you leave the carshopleavecarshop.sqsplayer removeaction buyredcar
I hope this will be alright, but notice you have not enough money now to buy the car, you can set the 100 in the init field to 400 or more, then you can buy the car.
make sure you name the car, which you want to buy, "redcar"
I think this was it all, if you have Q, ask me or Kevin, because he made this script and I stole it ;D