Read actmyname's post on shorena's post above on how to sign a message.
Below is a signed message as you asked:
13CZj5mHbrh3mAyjBZ3yCfKKeFUsfsrDC9Date: 9th Oct, 2016...
Hi, this is gkv9 and I am taking a loan of BTC 0.07 from DarkStar_...
As decided, I will be repaying in two weeks from now and the amount will be BTC 0.08, and if I fail to repay, we will proceed as discussed in PM...
Best regards,
gkv9...

IANO4jTLHih2k4qhqno5pLLxqAknwcgCR36vPXSX0KkVYbY2Kheqpvy0XADj6hQ0dfz9JUE3QOka9zj
7HenrBwM=
Please send the coins to the mentioned address...

I'm taking one more look into this loan, will fund afterward.