When ever I have toast scraps over after breakfast I always give them to my dog as a treat. I ask him to sit twice and if he doesn't do it I tell him if you don't do it then your not having it and I walk away. (I do realise he doesn't understand me and I'm saying it for my own benifit). After I have walked away I leave it for a few seconds and try againn and tell him to sit twice and this time he does it. What I didn't realise at the time was I have trained him to do this and the walking away and coming back is what I do. LOL.

Today after the first time of asking him twice and he didn't do it I walked away and thought if your not going to sit your not having it and threw the toast in the bin. Shortly after I had a thought have I trained him to do this? So I did a test and I have trained him to not sit when first asking, I walk away for a few seconds and I come back asked him to sit and he does sit.

So my question is how do I untrain him and how to get him to sit on the first asking.

Just ask once then walk away, he will very soon un-learn !
Just make sure you don't ask again for a good 15 mins especially if it's for a tidbit.....You can ask once then just wait till he offers the sit and this will work well.

Thank you Merlot.

How about not giving him toast left-overs at all?   Eating this isn't essential and will probably only make him fat

What Merlot said!

This is a common thing - I am forever telling people not to repeat cues multiple times because dogs can and do learn to only do it on the 3rd (4th, 5th...) time!

Had a Lab that used to come to class who learned to count up to Six !