From Longman Dictionary of Contemporary Englishwrite back phrasal verbANSWER/REPLYto reply to a letter that someone sent you, by writing a letter to them I sent them a card once, but they never wrote back. to I wrote back to them immediately, thanking them for their kind invitation. → write→ See Verb table
Examples from the Corpus
write back• She didn't really expect him to write her back.• When men write to me and try to help me, then yes, sometimes I write back!• Sara wrote asking if she could help, so I wrote back and said yes.• Professor Sano writes back by return mail.• I was never able to write back, I tried writing to Home Truths and a Birmingham paper to no avail.• Those that did write back stated that they were very happy with their present designs.• Josh, have you written back to Grandma yet?• Wilson did not write back until 1st February, 1858.• He wrote back with a brief autobiography.From Longman Business Dictionarywrite something → back phrasal verb [transitive] ACCOUNTING to increase the value of an asset that had been written off or written down. This happens, for example, when a debt that you thought would not be paid is unexpectedly paid → see also write-back → write→ See Verb tablewrite-backˈwrite-back noun [countable, uncountable]ACCOUNTING when the value of an asset that had been written off or written down increases, for example when a debt that you thought would not be paid is unexpectedly paid 