Complex sentence: Sending academicians abroad was quite rare at that time (as opposed to the early years of the Russian Academy of Sciences), so Lexell willingly agreed to go for a trip and was offered to write an instruction for himself which then without changes was signed by.
Split into simpler sentences: Sending academicians abroad was quite rare at that time (as opposed to the early years of the Russian Academy of Sciences), so Lexell willingly agreed to go for a trip. He was offered to write an instruction for himself which then without changes was signed by.