The application for a K1 Fiance Visa and/or CR1 and IR1 Marriage Visa requires the Petitioner (US Citizen) to prove that he or she has the income to support the application. In the case of a Fiance visa, the Petitioner must file an Affidavit of Support using form I-134. For a CR1 or IR1 Marriage Visa, the form is an I-864 Affidavit of Support.
Unfortunately, in the case of a Fiance Visa, you cannot use the foreign fiancés (also known as the beneficiary) income to help meet the financial requirements of a Fiance Visa application. For this visa, only the Petitioner or a close family relative of either party, acting as a co sponsor, can be used to provide evidence of income to meet the Fiance Visa application requirements.