The striker was presented to his new club's fans on Thursday night after completing a move for an initial 7.5million euros (£6.5m).
But the manner of the negotiations angered Del Nido, who told his club's website: "We were treated badly and that's putting it mildly.
"The fee was 7.5million euros plus add-ons, payable in several instalments. They demanded payment up front at the last moment.
"That was just one of many 'adventures' in this affair. One problem followed another. Fortunately, the player really pushed to arrange things.
"It's the worst transfer (negotiation) I've had to work on in my career."
PSG defended their conduct in a statement on their official website on Friday morning.
It read: "Paris Saint-Germain respect the rules of courtesy and professionalism in all negotiations.
"When it comes to player transfers, it is legitimate and normal to demand bank guarantees from the buying club.
"If they encounter difficulties in obtaining a part of the sum due, we propose as an alternative payment upon signing. It was this solution which was taken by the president of Sevilla FC, Mr Del Nido, in the transfer of Kevin Gameiro.
"Paris Saint-Germain have done everything to facilitate the move and allow Sevilla to add Kevin Gameiro to their UEFA (squad) list."