Section 1. Section 6 of chapter 62 of the General Laws is hereby amended by adding a new subsection (s): - (s) In the case of an individual whose gross income is less than 200 percent of the federal poverty level and who pays rent for his or her principal residence and does not receive any form of rental subsidy that varies with the amount of income and such residence is located in the commonwealth, a credit shall be allowed against the taxes in an amount equal to 50 per cent of such rent; provided, however, that such credit shall not exceed $3,000 for a single person, for a person that qualifies as a head of household under section two (b) of the Code, or for spouses, and, provided further, that one claiming such credit may not also claim the deduction otherwise allowed by subparagraph (9) of paragraph (a) of Part B of section 3 of chapter 62. If the amount of the credit allowed hereunder exceeds the taxpayer's liability, the commissioner shall treat such excess as an overpayment and shall pay the taxpayer the amount of such excess, without interest.