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.
The purpose of a K1 Fiance Visa is to provide a visa to the foreign finace of a US Citizen who wishes to get married. Its a benefit of US Citizenship. When processing the visa application, the US Government through The US Department of Homeland Security and the US State Department will review the application to determine the truthfulness or validity of the relationship. Dishonest relationships will be rooted out and those applications denied. The responsibility for providing proof of the relationship is on the couple applying for the visa. In determining what evidence is helpful in proving your relationship is genuine, you must consider what an honest relationship looks like. A couple in love do things together. They go places together. They talk and share information about themselves with others. If you focus on just those basic relationship areas alone, you can come up with plenty of evidence to support your application. Here is a list of evidence that that touches upon each of those basic relationship facts:
Marriage Visa Attorney Blog