Section 50C provides that if the value stated in the instrument of transfer is less than the valuation adopted, assessed or assessable by the stamp duty authorities, the valuation as adopted, assessed or assessable by the stamp duty authorities will be considered for the purpose of computation of capital gains arising on transfer of land or building or both.In your question the value of the land is stated at Rs. 7 lacs but according to the stamp duty authorities the valuation of the land is Rs. 13 lacs, then it will be considered that the land has been sold for Rs. 13 lacs and capital gains will be computed on the basis of Rs. 13 lacs.
Exception is provided in subsection(2) which prescribes that in a case where assessee claims before the AO that the value adopted or assessed or assessable by the Stamp Valuation Authority exceeds the fair market value of the property as on the date of the transfer and the value so adopted or assessed by the Stamp Valuation Authority under sub-section (1) of section 50C has not been disputed in any appeal or revision or no reference has been made before any other authority, court or High Court, then the AO may refer the valuation of the capital asset to Valuation Officer of the Department. Thus, according to sub-section (2) in a case where assessee claimes before the AO that the value adopted or assessed by Stamp Valuation Authority is exceeds the fair market value on the date of transfer and assessee does not dispute the valuation done by the Stamp Valuation Authority in any appeal or revision or reference then AO may refer the valuation of the said property to Valuation Officer and as per sub-section (3) in case where valuation done by the Valuation Officer is less than the value adopted by Stamp Valuation Authority then the valuation done by District Valuation Officer shall be taken as full value of the consideration received or accruing as a result of this transfer.