Where i can found a font like this free? i used "Whatsmyfont" and it is "FF DIN OT Cond Medium" but it isn't free...

This is a commercial font

http://www.dafont.com/tgl-0-1451-engschrift.font?af=on&psize=l&text=LE+SAMUEL

Also free. In the 2005 series the Roadgeek 2005 Engschrift.
http://www.n1en.org/roadgeek

That's not a free version, that's a different font but whatever.

Roadgeek is free for personal use. OP never mentioned commercial use.

Does the author may demand payment? How to prove that a different font?

They are only the same when both the file-sizes and the CRC32 checksums are identical.
The font in question in this thread is originally the Engschrift and Mittelschrift, later (Deutsche Gründlichkeit) named DINxxxx (Deutsche Industrie Norm).
The copyright on the letterform design has expired so anyone can make a font following the Engschrift and Mittelschrift letterforms. One may not take an existing font file, change the naming section and present it as a new font. The first is called copy-cat etc. the latter is cloning, theft.