stupid-questions:
What I now believe from studying my TEA5767 library, is that it accepts floats, not chars (or in that case, char arrays)
. . .
Here is an example which appears to use the same library for the TEA5767 as you are using. Clear is that the frequency is a float:
void setFrequency(float frequency) Sets the frequency in MHz.
You have also to decide how the user is going to enter the frequency through the keyboard, for example, are you going to require the decimal point as in '101.3' or is the user simply to enter '1013' and the decimal point is assumed ? You probably don't want more then 2 decimal places but that depends on the FM channels in your area.
Once you have decided that, then you can look for a routine to convert the text input into a float number.