If i remember correctly the RB523 is a small block motor, it designed for the Traxxas REVO. The REVO is 1/10th scale not 1/8th.
I would take it back to him and get your money back, they should know it doesn't fit.
just found it on there webby
http://www.rbproducts.com/mainEn.php...T&Series=TM523