The designer in question is Project Runway's Michael Costello -- and Floyd's baby mama Josie Harris tells us, Costello was hired to design a very special dress for the boxer's 10-year-old daughter Jirah to wear at the Canelo fight in Vegas (left).
Josie says the dress came out great -- but when they showed up to the fight, she noticed Floyd's on-and-off-again side piece Shantel Jackson wearing the same exact dress (right).
According to Josie, she paid Costello $795 for a one-of-a-kind dress, and now she's pissed the guy gave a replica to none other than Floyd's girlfriend ... who also sat ringside.
Josie tells us, "It's an unspoken rule and a cardinal sin for a 'real designer' to dress clients alike going to the same major event ... Tacky, tacky, tacky."
And Josie didn't stop there, turning her anger toward Jackson as well ... adding, "She is a total gold digger ... She conveniently came back around 2 days before Floyd got his $41.5 million check."
As for the designer, Costello says he will NOT be giving anyone a refund -- telling us, "No exclusivity was promised regarding the dress. If you want exclusivity, you need to pay more." Costello says his people told Miss Jackson that Floyd's daughter would be wearing the same dress and she didn't care.