If it is an informal memo, then the "style" isn't so important. However, to use a proper style, the "Sub" line should probably read "Subject:" or "Re:" and follows the salutation. (Yes, that seems odd, but true.)
Your second sentence is unclear. You say "required" - was that a prerequisite to the testing procedure? In which case, why tell him about it now; he has passed the test. If you mean that you need more of the substance before you can proceed, then you need to say "We require an additional 500 grams...." [Grams, by the way, is not a proper noun and should not be capitalized in this sentence.]
Then the "... manufacture small batch to conduct stability test before going into regular import at an early date" might be more clearly stated as "... manufacture a small batch to conduct a stability test before we are able to go into regular import."
Your sign-off of "Thanks & regards" is very informal, and therefore inconsistent with the Dear Mr in the salutation. I would suggest, "Yours truly," (and it does call for a comma).
If you want to add a polite filler, then insert another sentence, such as "Thank you for your attention to this request." or the like, before the "Yours truly" line.
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