The Nigerian spent a year on loan at United from Shanghai Shenhua, scoring five goals in 23 appearances, before leaving for Saudi side Al Shabab in January.
Ighalo fulfilled a lifelong dream playing for the Red Devils, which initially motivated him to reject Spurs.
In an interview with Sky Sports, Ighalo said: "That was close - my agent called me and told me they had offered to match whatever I was earning but once I heard Manchester United were interested, there was no turning back.
"I couldn't think about anything else other than making the Manchester United deal work. I was going to have to take a pay cut but I didn't care, I just wanted the deal to happen.
"I just wanted to play for Manchester United. It's my dream club and it was always going to be a privilege for me to play at Old Trafford.
"I'm just so grateful to God that it happened. Every day I wake up and I know that I am an ex-Manchester United player. It is something that I will never take for granted."