Or HaChaim on Exodus 11:1/אור החיים על שמות י״א:א׳

1 ויאמר ה׳ ..עוד נגע אחד, G'd said to Moses: "one more plague, etc." This chapter tells us about matters G'd had already mentioned to Moses earlier while Moses was still in Midian. At that time (3,19) G'd had informed Moses that Pharaoh would not let the Israelites depart even after He applied a strong hand against him. In 4,22-23 G'd had already warned Pharaoh of the consequences of not letting the Israelites depart. You will observe that the end of the cycle of nine plagues left Pharaoh exactly as obstinate as he had been at the beginning. He had started out by not even considering the demands of G'd, and though he appeared to have softened his stance, making progressively greater concessions, he was now back to square one having ruled out the departure of any of the Israelites. In 7,4 G'd had told Moses that the sign that the plague of the killing of the firstborn was about to occur would be Pharaoh's refusal to listen. As soon as Moses heard that he was not to appear again at court he knew that the time had come for the last plague after which the Exodus would begin. Moses knew that this plague would be the most severe because G'd asked him to instruct the people to "borrow" as many silver and golden trinkets as they could. The word עוד then does not mean an additional plague as much as it means "the plague I have already spoken to you about once before, I am mentioning again." This explains why Moses did not wait after verse four until G'd described the plague to him, though G'd had done so every other time when Moses was told to warn Pharaoh. Moses proceeded to inform Pharaoh on the spot. Whereas it is true that we do not find that G'd had previously told Moses that the plague of the killing of the firstborn would occur at midnight, we may assume that G'd had told Moses that detail also at that time, but that the Torah did not record it there as it had not yet become relevant. This is why it is not difficult to understand that G'd repeated here what He had told Moses originally, i.e. that the Jews would take with them the silver and gold trinkets borrowed from the Egyptians as well as their garments and that they would "empty" Egypt (Exodus 3,22).