Potential buyer disagree to buy property and asks to return money

Dear Sir, I am NRE residing in Jeddah, Saudi and I was owning a small residential property (around 4 cents) at Kerala. I put this land on sale for 20 Lakhs about 1 year before (October 2015) due to some urgent cash requirements. Accordingly one person shown interest in this property on 1 November 2015. The potential buyer is also an NRE residing in Manama, BAHRAIN. He sent his representative to see the property, they liked and agreed for registration by December 2015. They have collected the copies of registration documents for their references. After all the negotiations we agreed for a total amount of 19.2Lakhs. As a surety for this deal I have asked him to pay some cash for the confirmation. He keep on delaying the payment saying we will finalise the registration by Mid December 2015. Since I live outside the country I have processed power of attorney for my father to do the registration on my behalf in December. During Mid November his father (NRE - residing in Jeddah, Saudi) visited the plot, was happy and agreed to do the registration at the earliest. Upon his (father) return to Saudi, he came to my home in Jeddah and handed over his (Father) cheque (postdated 05 December 2015) amounting 1Lakh as a surety for the deal and agreed to do the registration by Mid December. I have sent this cheque to my home in India for encashment. Later he (Father) told me that his son (potential buyer) will transfer the cash from Manama directly to my account and don’t submit the cheque. As agreed, I have asked my parents to return the cheque back to Jeddah for giving to his father. The potential buyer himself went to see the property in December and he confirmed that he is happy with the plot. Upon his return to Manama, BAHRAIN, he said the registration should be postponed to February 2016 due to his cancelled leave and some urgent job related issues. He transferred an amount of 1Lakh to my account on 21 December 2015 for surety. On January 9, 2016 his brother visited the plot and noticed that an overhead power line (66kv) was present on the plot and told me that they won’t be able to build a house in there due to this cable. I have told them that this cable is passing through the north east corner of my plot and there is no issues with the house construction and people are living/constructing houses in nearby plots with the same situations. This cable was there from the beginning and visible to anyone who visit the property. Moreover this was clearly written on the registration document and a copy of the same was given to his representative in November 2015. He said he will talk to his family and some architects about this issue. They went on silent for a month and on 6 February 2016 they have told me to search for another buyer and they have wasted my 3 months. Since then I was keep searching for another buyer. To my luck, I found one buyer recently and sold the property to him (on 3 November 2016). I was forced to sell much lesser amount due to the urgency and current market conditions in Kerala. The previous buyer’s (potential buyer) father approached me yesterday and asked me to return his cheque (1Lakh) and the amount (1 Lakh) his son paid. He also threatened me that he will complaint against me to authorities if not paid. It seems he came to know that I have managed to sell this property to another person. I would like to clarify regarding the 1Lakh payment and the submitted cheque, do I need to return his money and cheque? Please note that I have not engaged any agreement or documentation with them. All communications were held via whatsapp, phone calls and personal meetings. 1)Can I consider the amount paid against the issues (Time delays and sold for much lesser money) caused by them? 2)Is there any valid legal claim for them to put a case against me? 3)Is there any problem if I stop answering their calls? Regards, John