User blog comment:B214/Talk About What You Want! 3/@comment-25290120-20141125182448/@comment-1359322-20141127111942

Not really, steal in this case refers to a bargain meaning paying less than she actually has to pay. When Rias reincarnate Issei, she paid the full price in this case 8 Pawn Pieces, no discount.

In both Yuuto and Saji's case, the two of them are reincarnated at a cheaper price than their current worth. If Rias and Sona reincarnated reincarnated Yuuto and Saji later, they would have to pay a bigger price in this case, two Knight Pieces for Yuuto and More than 4 Pawn Pieces for Saji. In Issei's case regardless of his past or current worth, it'll still be 8 Pawn Pieces, so even if Issei's Pawn Pieces has become Mutation Pieces, Rias didn't gain any bargain. Rias wouldn't have save any additonal pieces like she and Sona did with Yuuto and Saji.