The article deals with declensional forms of cardinal numerals attested in the manuscript business documents of the first half of the 17th century from the Pskov region. Many declensional paradigms were in the process of change in this period. Some cases exhibit variant forms, which can be classified into archaic, transitional and new. While the instrumental falls together with the dative in the nominal and pronominal declension where the form becomes dominant (e.g. за рукам своим), in numerals the dative is replaced by the instrumental (e.g. тридцатьма, пятьюдесят). This is due to the fact that numerals resist developing a uniform oblique case, which would have emerged if the dative of the type пяти had taken over.