Promoting MySQL slave with unknown replication delay
Master + Slave?  Not Dual Master?

With Dual Master, each server is the Master and Slave to the other.  In this case either can receive writes, but you should write to only one.  More pertinent, failover and failback is readily available.

With Master + Slave, you have not prepared for the failback because things like log_bin and log_slave_updates are not turned, nor is the Master listening to the Slave.

I'm afraid your only option (now) is to stop the Slave (which you promoted to Master), copy the data, build a Slave to it (presumably on the original Master's server).  That is, you lost the Master, and whatever data is on it is useless.  It's like initially adding a Slave to a running system.