The 28 sweaters need 28 x 4 = <<28*4=112>>112 balls of yarn.
So, Chantal will spend 112 x $6 = $<<112*6=672>>672 in all for the balls of yarn.
Her total earnings for selling 28 sweaters is $35 x 28 = $<<35*28=980>>980.
Therefore, Chantal will gain $980 - $672 = $<<980-672=308>>308.
#### 308