Is it legal to use the money paid for a condo a foreigner sold to buy a new condo or the money must be transferred abroad first and re-sent back to Thailand to get a new FET?

Why not? It's your money.

Is the new condo more or less expensive? If less expensive, and you still have that slip that original purchase price was transferred, you can use that. If new condo is more expensive, you'd need to transfer difference from abroad.

 

You can buy condo but can't transfer it unless you can show (with bank form/letter) that entire amount is transferred from abroad.

 

Or so I was told. But then got PR and didn't need to worry about that anymore.