Depending on how similar they are (i dunno how generic the ‘xxx’ is), the fact you approached them first and offered to sell it to them may be enough evidence that you registered the domain in bad faith and they could win a UDRP proceeding against you.
Generally it’s a bad idea to infringe on people’s trademarks and then try to sell it to them 
Only a lawyer could really answer if you made a stupid move, depending on how you phrased the email.