Better job vs MS? Which reason to give for leaving a org in 3 weeks

So this past month has been a hell of roller coaster ride. I was desparately looking for job in March and found a great org and joined it on May end. Now here I am realising most of my work is very clerical/commercial in nature. Now a great opportunity opened up in my prev org and my ex manager referred me there and its a great IT implementation role. This will help me in pivoting to IT and if I learn some coding I can even get into development. Meanwhile I have also received a MS admit from Europe and Loan is also sanctioned for the same. I intend to join the prev role as it would help me boost my career prospects but I am not able to decide which reason to give to my current employer for leaving? Should I tell the truth or lie about going for MS? Btw PF account is yet to be generated for new employer. Need your help and opinions guys