مُكَاتَبَةٌ

and ↓ تَكَاتُبٌ are synonym: (S, K:) you say, كاتب صَدِيقَهُ He wrote to his friend: and ↓ تكاتبا They wrote, one to the other. (TA.)

كاتبهُ, verbal noun مُكَاتَبَةٌ (Az, K, Msb) and كِتَابٌ, (Az, Msb,) (tropical:) He (a slave) made a written [or other] contract with him (his master), that he (the former) should pay a certain sum as the price of himself, and on the payment thereof be free: (K, &c.:) also he (a master) made such a contract with him (his slave): (Az, Msb, &c.:) and ↓ تكاتبا They two made such a contract, one with the other. (Msb.) The slave in this case is called مُكَاتَبٌ (S, Msb) and also مُكَاتِبٌ; and so is the master; the act being mutual. (Msb.) [But the lawyers in the present day call the slave مُكَاتَبٌ only; and the master, مُكَاتِبٌ.] الكِتَابَةُ, signifying “ what is written, ” is tropically used by the professors of practical law as synonym with المُكَاتَبَةُ, because the contract above mentioned was generally written; and is so used by them when nothing is written. It was thus called in the age of el-Islám, according to Az. These two words are said by Z to be synonym; but it is thought that he may have written the former by mistake for الكِتَابُ, adding the ة by a slip of the pen. (Msb.)