Sometimes people just say 'close' to refer to closing a deal or sale: "Did you close?" However, these variations, 'seal the deal', 'close the deal', or 'close the sale', all mean the same thing. People don't really say 'seal the sale', so you would sound strange if you said that. 'Seal the deal' is slightly less formal, and you might not want to use it in a highly professional situation. It can also have entirely different connotations in Slang usage, as sometimes to 'seal the deal' means to have sex with someone (as if one participant had been 'negotiating' his or her way towards this physical act). So be careful how you use it!