It would not be considered natural in everyday English usage, but in international scientific usage I think it is acceptable. In that rare context "pay back" just means to add sufficient distilled water to bring the volume of the solution up to 10 ml.
Having said that about the English, I am in some doubt about the math. Can you really dissolve 10 mg of gallic acid in less than 10 ml of water? Maybe, but it seems unlikely to me. And if you could and did, would the resulting solution be less than 10 ml in volume?
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