Or HaChaim on Exodus 8:5/אור החיים על שמות ח׳:ה׳

1 התפאר עלי, "have this glory over me, etc." What exactly did Moses mean by the expression התפאר עלי? If what he meant was that even if Pharaoh would ask for an immediate removal of the frogs this would constitute his glory, the result of Pharaoh asking that the frogs be removed only on the morrow would indicate that he was not interested in besting Moses; Moses' confirmation: "as you have said," would make little sense in that scenario. We also need to analyse why Moses did not say מתי אעתיר לך which we would have expected, but said למתי וגו׳?

2I believe Moses wanted to demonstrate that G'd tries to oblige those who are dear to Him so that it was beyond his imagination that G'd would forsake him (not do what he promised). He did this by contrasting the relationship between man and G'd with that of slave and master. When a slave asks a favour from his master he will be happy if his master grants the favour at all and it would not occur to him to attach conditions as to the time when such a favour has to be granted. He will be even less likely to ask for the date of such a favour to be advanced or delayed, but will await the time when he judges his master to be in a favourable frame of mind to grant his request. He will be most happy if his request is granted at the time it pleases his master. Not so the relationship between Moses and G'd. Moses was anxious to demonstrate to Pharaoh that a) he could offer a prayer at any time; b) his prayer would not only be granted but that he could attach conditions as to the "when" of G'd's response. This is why he asked Pharaoh what time or day he wanted him to pray that the frogs should disappear. Pharaoh was to prove himself superior in that he could determine when Moses should pray.