Converting a float to hours:minutes in Numbers

You can use the VALUE() function for that. Its only argument is the cell that you want to evaluate

From the documentation:

The VALUE function returns a number value, even if the argument is formatted as text. This function is included for compatibility with tables imported from other spreadsheet apps.

VALUE(source-string)
source-string: Any value.

Notes
You’ll never need to use the VALUE function in a new table, because numbers in text are automatically converted for you.

Only the formatted text is converted. For example, if you type the string $100.001 into a cell, the default format will display only two decimals ($100.00). If VALUE refers to this cell, it will return 100, the value of the formatted text, not 100.001.

If the argument can’t be returned as a number value (does not contain a number), the function returns an error.

Examples
=VALUE(“22”) returns the number 22.

=VALUE(RIGHT(“The year 1953”, 2)) returns the number 53.

Although one cannot style individual characters in a formula, one feature of Apple Numbers is the canvas. One can take advantage of this by adding tables and text objects anywhere.

The above screen shot uses a static (as the OP's "Invoice terms" text is hard coded in their formula example, it is assumed static) text field placed next to the table (a single cell table with no border, in this case) containing the cell with the formula.

The OP's use of the CONCATENATE function is unneeded as the ampersands do the same job. To use CONCATENATE in this case:

=CONCATENATE(Terms,", please pay by...")

Where Terms is the reference to the cell containing the Terms text.

Detailed screenshot:

By using a number for the days, this can be used in a calculation against the Invoice date to determine the date payment is due. But that is a topic for another question. ;)