Silly question on long term capital gains

Guys a quick question: I own an apt in which I reside. I purchased a plot and constructing a house with my savings. I sold an empty plot owned by my mother because of which I need to invest 70 lacs appropriatly to save 20% tax. Can my mother buy my apt I am living in for 70 lacs. What ever little LTCG I incurr can be offset against house I am constructing. This way I will loose only around 5% as stamp duty instead of 20%. I can later return 70 lacs to my mother as it does not attract gift tax. Is this possible? If possible, it is like allowing section 54f for investment made in sons name, ofcourse at the loss of stamp duty. Am I missing something? Also I have a joint account with my mother. I am using at my will for my purposes by signing on checks or by wire transfer. Credits to this account are from mothers funds. I am doing this as there is no gift tax between us. Am I doing something wrong as AO can interpret! Thanks for replies....