Yawgmoth's Will
Artist: Ron Spencer

seen from Canada
seen from Indonesia
seen from Malaysia
seen from United States
seen from United States
seen from United States
seen from Hong Kong SAR China
seen from United States
seen from China
seen from United States

seen from Lithuania
seen from United States
seen from Argentina
seen from T1
seen from China

seen from United States
seen from China

seen from United States
seen from United States

seen from Australia
Yawgmoth's Will
Artist: Ron Spencer
Yawgmoth's Will
Artist: Ron Spencer TCG Player Link Scryfall Link EDHREC Link
Yawgmoth's Will
Artist: Ron Spencer TCG Player Link Scryfall Link EDHREC Link
So I was wondering: if I grant cards flashback via Past in Flames (or a card has flashback normally) can I discard cards to cast them from my graveyard via Dream Halls' effect? Thanks :)
No.
You can only pay one alternate cost to cast a card, and the only option you have to cast those cards from your graveyard is to pay their flashback costs.
If you were given permission to cast the spell through an effect that doesn’t specify flashback, like Yawgmoth’s Will, then you could discard cards to cast them with Dream Halls.
Yawgmoth’s Will extreme altered by Ron Spencer :)