We know that Alan forgets his wallet on purpose so when it's time to pay, he does his usual act.
Alanchuckle) Oh, would you believe it? I forgot my wallet.
Thus he leaves Lydnsey to pay for everything and it annoys her. He did it before with Charlie until Charlie discovered that Alan was squirreling away money for his old age because he believed Jake would be incapable of taking care of him.
Lyndsey should say,"Alan Harper, if you can't afford to even pay for your half of dinner, then you can't afford to have a girlfriend, either!" and she should then storm away.
If Alan leaves his wallet behind when he goes out with Lyndsey, then that means he probably leaves his driver's license behind, too! This should happen, Alan and Lyndsey are on their way to dinner and an officer pulls them over because he's noticed one of the MANY problems with Alan's car and asks to see his license and registration.
Alanchuckle) Oh, can you believe it, officer? I forgot my wallet.
Officer: You mean to tell me that you've been driving around without your license?
Alan: (smiling with his eyes wide open) Yeah, can you believe it?
Officer: Actually, no.
Officer: Because most people usually don't forget their wallets when they go driving and when they do they act so surprised and nervous, but you seem very calm. Almost as if..this wallet thing was well rehearsed.
Lyndsey: Or maybe it's because since you've forgotten your wallet so many times already, that you're used to it by now.
Officer: Ma'am, you mean he's done this before? (over the radio) I got a violator here. Driving DELIBERATELY without a license and apparently on several occasions.
Alan would then be arrested but Walden would bail him out. Later Alan and Lyndsey would be on their way to dinner and a cop pulls him over.
Officer: License and registration, sir.
Alansighs and pulls out his wallet to show his license) Here. Was I speeding or something?
Officer: No, I just wanted to make sure you had your license, carry on.
Alanhumilated) I can't believe this, ever since my arrest, a cop has been pulling me over just to make sure I have my license.
Lyndsey: Well, at least now you won't ever forget your wallet.
Alan: That's right because as you can see, I have it right here.
Lyndsey: Good. Oh, and when it's time to pay for dinner, you better tell me that you forgot to put money in your wallet.
Alanopens his eyes widely) Uh,... you know I'm actually not that hungry. What do you say we just go for a walk or something.