The only way to get your money back from a masspay payment is to say you sent it to the wrong person & it was paid in error and you will have to possibly sign a statement that says this and fax it to them or email to get the money back. If the person is'nt giving you your domain then this should not be an issue.
I have only had to reverse one transaction ever made by masspay (to a member here for 1500 recently, & you know who you are f*ckface) and paypal was very helpful and attentive to my needs so I did not have to make some phone calls and resolve it the old fashioned way. I do carry a decent balance in my account though and have been a customer of theirs for many many many years. For the most part it is VERY safe to receive masspay payments...they are much harder (nearly impossible) to reverse than credit card based ones. Most scammers are not going to have money sitting around in their paypal accounts and send out masspay payments to you. Always keep all important emails saved from every deal you do so that you can refute any possible claims in the future regarding money/transfers. They will ask for them in any situations regarding a claim.