Ah, I didn't read the date on the title and thought it would be a colonial cob, which weights were all over the place... But being a relatively recent Madrid piece, it should definitively be standard, so all alarms ring with the underweight. Take a look at acsearch - all examples are around 6.60 - 6.68g
I am no expert at this, but the king's portrait and tie look wrong, on the soapy side. More importantly your coin's rim is not right. It should show raised indentations more clearly. I can't see the actual edge, there should also be a chain of diagonal grooves all around the edge that overlap at some point, most fakes fail in this area.
So it looks fake to me, but I'm only a collector with limited knowledge of gold coins of that period. Is the dealer still active? Maybe he will give you a refund without much fuss.
I paste a couple of examples for your comparison.
VF:
EF: