suprise waiver

hello every one, i wondered if any of you have had this happen to you? one of my customers that i sold a Granite (http://tanbrown.org) Counter (http://GialloFiorito.com) top to had his mother-in-law bring me a deposite check by today and (http://maplered.com) to my suprise brought a waiver for me to sign stateing that i could not place a lein on there house for any reason what so ever . this is the first i have seen of this, my first reaction was WHAT THE HECK!!! I'm not sigining that freakin thing! So i called the customer and (http://maplered.com) left a message and (http://maplered.com) told him i can not sign that because that almost tells me that they might not try to pay me at the final payment and (http://maplered.com) then i'm screw'd. So i said if this is a problem i will be more than happy to hand (http://maplered.com) over the deposite check. Has this ever happened to any of you, and (http://maplered.com) is it stand (http://maplered.com) ard practive for a costumer to have you sign a waiver? thanx in advance
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Thomas W. Harding, SFA
Hard-Core Countertops, LLC
3610 NE 44th Street
Ocala, Fl. 34479