Bond after EOL for study

I worked as Manager( Electronics) in Airports Authority of India (AAI), Ahmedabad. I joined AAI in august 2011. In the year 2018, I asked permission from AAI to pursue higher studies in abroad. My study specialization was the domain of my work. I propose to use my earned leave+ HPL ( approx 7 months) and rest ( 6 months approx) study leave without pay. I applied in May 2018 and got permission on 20th September 2018 with the clause of bond of three years service or pay to AAI 6 month pay. I received sanction order before one day of my flight. I opposed the bond verbally but I was said that if I will not give bond, I cannot leave. My senior officer described that they are asking bond to maintain my seniority. I had deposited ten lacs rupees as tuition fee to my university, so I submitted bond and went to France for study. As promised, I joined back to AAI on 23rd October 2019. When I joined, I came to know that I am not promoted too even I was eligible in seniority list. 50 manager below me have been prompted too as senior manager. I represented to higher authority to cancel the bond since they have manipulated clause mentioned in AAI leave regulations. The regulation says: " Where a Government servant is granted extraordinary leave in relaxation of the provisions contained in Clause (e) of sub-rule (2), shall be required to execute a Bond in Form 6 undertaking to refund to the Government the actual amount of expenditure incurred by the Government during such leave plus that incurred by any other agency with interest thereon in the event of his not returning to duty on the expiry of such leave or quitting the service before a period of three years after return to duty " . This clause is also not applicable in my case since, I have not availed leave in relaxation to provision mentioned. Additionally, there was no expenditure occurred for me, hence 6 months basic +DA salary clause is not justified. I requested you to relook into my case. and cancel the bond clause mentioned in sanction order. Till today, I have no reply. I was very frustrated. Even if I will give representation for promotion, they will not promote me from back date. In AAI promotion got effected from the date of joining. I could not handle the work environment where I am highly qualified and juniors to my juniors too. At last, on 3rd February 2020, I resigned from my service. I am sure that AAI will not cancel my bond and will ask to deposit the amount in few days. May you suggest me that if bond was legal or illegal as per your experience?